Depends on the age of the printer, but look at the serial port and you might find it's on a section which you can remove and replace with a new module (or there might be a section nearby that you can add a module)
Here's an example of another module for a different HP printer but if capable they should be very similar.
http://cgi.ebay.co.uk/HP-J3110A-Jetd...-/300554554149